The present invention relates to a tire pressure monitoring device and a method of monitoring tire pressure for a system including both a direct measurement tire pressure monitoring system and an indirect tire pressure monitoring system. The invention further relates to a computer program product for implementing the method.
It is of great significance for vehicle safety to reliably monitor the tire pressure on all wheels of a motor vehicle. There are different approaches how to realize tire pressure monitoring systems. So-called tire pressure monitoring systems with direct pressure measurement, as described in application DE 199 26 616 C2, exist which determine the respective pressure in the associated wheel by means of pressure sensors in the individual tires. Systems of this type monitor the tire pressure on all wheels independently, yet they are relatively expensive as they require additional devices, e.g. For transmitting and evaluating pressure sensor information. Further, so-called indirectly measuring tire pressure monitoring systems are known, e.g. From DE 100 58 140 A1, which can detect pressure loss based on auxiliary quantities, e.g. By comparing the rolling circumferences of the individual wheels.
These systems suffer from the disadvantage that a defective tire will only be detected at a significant pressure loss. Admittedly, systems of this type are inexpensive and reliable, yet they function only if pressure loss occurs on one wheel. If pressure loss occurs on several wheels at the same time, this condition will not be detected.
DE 100 60 392 A1 discloses a tire pressure monitoring device which comprises a combination of a tire pressure monitoring system with indirect measurement and a tire pressure monitoring system with direct measurement. The task of the tire pressure monitoring device described in this publication is to monitor inflation pressure loss on all four wheels by the combination of a tire pressure sensor and a tire pressure monitoring system with indirect measurement.
It is disadvantageous in this respect that when using only one tire pressure sensor, the wheels on which no tire pressure sensors are mounted can only be monitored with relatively high detection thresholds. The consequence is that inflation pressure loss is detected at a very late point of time only. It is achieved by the alternative use of two tire pressure sensors as mentioned in the above publication, with exactly one tire pressure sensor being arranged on each vehicle axle, that individual tire pressure nominal values can be determined for each axle. However, this provision does not lead to a considerably earlier detection of inflation pressure loss. As a tire pressure monitoring system with indirect measurement operates on the basis of rotational wheel speeds and, hence, is directly dependent on the wheel rolling circumference, pressure loss on the driven wheels can frequently be detected only very insufficiently or in rare moments of their free rolling.
When using a tire pressure sensor on only one wheel of the driven axle, it is only possible to detect very great pressure losses on the other driven wheel. Besides, there is still the problem that wheel slip on a driven wheel can be interpreted as pressure loss on this wheel by the tire pressure monitoring system with indirect measurement because the tire pressure monitoring system with indirect measurement does not identify whether the wheel speed increase is due to a defective tire or a slip situation. For reasons of rigidity, it is therefore possible in a tire pressure monitoring system of this type to use only high detection thresholds for pressure loss detection.